FIG. 4 is a perspective view, partially broken away, illustrating a distributed feedback laser (hereinafter referred to as DFB laser) having a buried heterojunction structure (hereinafter referred to as BH structure), which is disclosed in, for example, ELECTRONICS LETTERS, 21 May 1987, Vol. 23, pp. 546-547 and ELECTRONICS LETTERS, 2 Feb. 1989, Vol. 25, No. 3, pp. 220-221. In the figure, reference numeral 1 designates a p type InP substrate containing Zn as a dopant. A double heterojunction structure including an InGaAsP active layer 2 sandwiched between a p type InP lower cladding layer 11 and an n type InP first upper cladding layer 12 is disposed on the InP substrate 1. An InGaAsP layer 13 having a diffraction grating structure 14 is disposed on the first upper cladding layer 12. An n type InP second upper cladding layer 15 is disposed on the diffraction grating structure 14. A first p type InP layer 3, an n type InP layer 4, and a second p type InP layer 5 are disposed on opposite sides of the active region, producing a p-n-p current blocking structure. An n type InP third upper cladding layer 6 is disposed on the n type InP second upper cladding layer 15 and the second p type InP layer 5. An n type InGaAsP contact layer 7 is disposed on the third upper cladding layer 6. An insulating film 8 is disposed on the n type InGaAsP contact layer 7 except for a region opposite the active region and extends onto the side surfaces of the laser structure. An n side electrode 9 is disposed on the n type InGaAsP contact layer 7 and the insulating film 8. A p side electrode 10 is disposed on the rear surface of the substrate 1.
A method for fabricating the DFB laser of FIG. 4 is illustrated in FIGS. 5(a)-5(f).
Initially, there are epitaxially grown on the Zn-doped p type InP substrate 1, the Zn-doped p type InP lower cladding layer 11 having a carrier concentration of 1.times.10.sup.18 cm.sup.-3 or more, the InGaAsP active layer 2, the n type InP first upper cladding layer 12, and the InGaAsP layer 13 (FIG. 5(a)).
After the epitaxial growth, the diffraction grating structure 14 is formed by a conventional technique (FIG. 5(b)). For example, using photolithography, a photoresist is deposited on the InGaAsP layer, exposed to interference fringes of light, and developed as a mask with a periodic structure. Then, the layer 13 is etched through the mask, and the mask is removed.
Thereafter, the n type InP second upper cladding layer 15 is epitaxially grown on the wafer, and a stripe-shaped dielectric film 20 is formed on the second upper cladding layer 15 (FIG. 5(c)).
Then, the wafer is etched using the dielectric film 20 as a mask to form the double heterojunction structure in a mesa shape (FIG. 5(d)).
Thereafter, the first p type InP current blocking layer 3, the n type InP current blocking layer 4, and the second p type InP current blocking layer 5 are successively grown on opposite sides of the stripe mesa by LPE (Liquid Phase Epitaxy) or the like, and the n type InP third upper cladding layer 6 and the n type InGaAsP contact layer 7 are successively grown on the surface of the wafer (FIG. 5(e)). Carrier concentrations of the p type InP current blocking layers 3 and 5 are in a range from 1.times.10.sup.17 cm.sup.-3 to 1.times.10.sup.18 cm.sup.-3 and are produced by doping those layers with Zn.
Then, the laser structure is formed in a mesa shape by etching (FIG. 5(f)). Then, an insulating film 8 is deposited on the entire surface and the side surfaces of the laser structure and a portion of the insulating film 8 opposite the active region is removed to expose the contact layer 7. Finally, the n side electrode 9 is formed in contact with the contact layer 7 and the p side electrode 10 is formed on the rear surface of the substrate 1, completing the DFB laser of FIG. 4.
In the DFB laser thus fabricated, reactive current flowing outside the InGaAsP active layer 2 is reduced by the p-n-p current blocking layers disposed on opposite sides of the active layer 2. In addition, since the carrier concentration of the first p type InP current blocking layer 3 (1.times.10.sup.17 .about.1.times.10.sup.18 cm.sup.-3) is lower than the carrier concentration of the p type InP lower cladding layer 11 (1.times.10.sup.18 cm.sup.-3 or more), the resistance of the current blocking layer 3 is higher than that of the lower cladding layer 11, whereby the reactive current during the laser operation is reduced.
In fabricating the DFB laser of FIG. 4 according to the process steps of FIGS. 5(a)-5(f), although the epitaxial growth of the semiconductor layers on the Zn-doped semiconductor substrate 1 is always carried out under the same conditions, if the Zn dopant concentration in the substrate 1 varies from substrate to substrate, between substrate lots, threshold current and efficiency of the completed laser unfavorably vary, resulting in difficulty in manufacturing high power output DFB lasers with high reproducibility.
Threshold current density Jth and external differential quantum efficiency .gamma.ex of a semiconductor laser are represented by ##EQU1## where dAL is the thickness of the active layer, .GAMMA.v is the light confinement coefficient, L is the resonator length, R1 and R2 are reflectances at the front facet and the rear facet, respectively, JO is the current density when the gain is O, .beta. is the gain factor constant which is proportional to the lifetime of carrier, .eta.i is the internal differential quantum efficiency, and .alpha.i is the internal loss due to free carrier absorption in the crystalline layers on the substrate. In order to reduce the threshold current and increase the efficiency of the semiconductor laser, the internal loss .alpha.i must be decreased, and the gain factor constant .beta. must be increased by reducing non-radiative recombinations of carriers in the active layer.